Objective-C单例超类？

Question

I'm trying to write a class that I can subclass to have an instant singleton. Here's what I have so far. It works until one of its subclasses calls another via sharedInstance which causes a huge loop that eventually runs out of memory.

Any ideas?

static NSMutableDictionary *sharedInstances = nil;

@implementation Singleton

+ (Singleton*)sharedInstance
{
    [Singleton initSharedInstances];
    Class myClass = [self class];
    Singleton * sharedInstance = [sharedInstances objectForKey:myClass];
    @synchronized(myClass)
    {
        if (sharedInstance == nil)
        {
            sharedInstance = [[myClass alloc] init];
            [sharedInstances setObject:sharedInstance forKey:myClass];
        }
    }
    return sharedInstance;
}

+ (void) initSharedInstances
{
    if (sharedInstances == nil)
    {
        sharedInstances = [[NSMutableDictionary alloc] init];
    }
}

@end

Answer 1

Why are you bothering with all this? If you're trying to enforce singleton behavior in the superclass by overriding -retain , -release , -retainCount , and +allocWithZone: then you're doing something completely unnecessary. Far more simple is just to provide a +sharedInstance method and do nothing else. If the user really wants to call +alloc / -init , they can, it just won't do them much good. For examples of this type of singleton in the frameworks, look at NSUserDefaults and NSFileManager (though in the case of the latter, these days Apple actually recommends you ignore the shared instance and alloc/init your own instances of NSFileManager ).

In order to do this simple shared instance stuff, all you have to do is, in the singleton class, implement the following:

+ (id)sharedInstance {
    static MyClass sharedInstance;
    static dispatch_once_t predicate;
    dispatch_once(&predicate, ^{
        //sharedInstance = [[MyClass alloc] init];
        sharedInstance = [MyClass alloc];
        sharedInstance = [sharedInstance init];
    });
    return sharedInstance;
}

Answer 2

I'm trying to write a class that I can subclass to have an instant singleton. Here's what I have so far. It works until one of its subclasses calls another via sharedInstance which causes a huge loop that eventually runs out of memory.

This sounds like you are describing mutual-recursion. If subclass1 calls subclass2 and subclass2 calls subclass1 then you need to break the loop somewhere, just as with simple self-recursion.

Your sharedInstance itself should not cause infinite recursion unless the init method you invoke itself invokes sharedInstance ...